Abstract
Abstract Tree health declines can be caused by interactions between pests and pathogens and many studies have shown a reduction in their damage in mixed species forests compared to monocultures. Yet few authors have considered tree diversity effects on both groups simultaneously. Moreover, it is unclear whether diversity effects on tree pests and pathogens are robust to changes in abiotic conditions, such as drought. We addressed tree diversity effects on foliar insect herbivory, oak powdery mildew and their interaction under contrasting water regimes in a large-scale tree diversity experiment in SW France. Using an irrigation treatment that alleviated drought conditions, we were able to experimentally assess the effects of tree diversity under contrasting abiotic environments. We surveyed plots along a richness gradient from one to four tree species, in which a focal study species of oak (Quercus robur) was mixed with other oak species (Q. pyrenaica and Q. ilex) and a taller, broadleaved species (Betula pendula). Increasing tree species richness lowered leaf miner abundance, leaf chewer damage and oak powdery mildew infection, consistent with a protective effect of resource dilution. However, richness effects on leaf miners were stronger in irrigated compared to non-irrigated blocks, indicating that environmental conditions can modulate diversity effects. Separate from the effect of tree species richness, the presence of birch in a plot increased damage by leaf chewers and powdery mildew, but lowered leaf miner damage, suggesting additional tree neighbour identity effects potentially linked to modulation of microclimate. We found a negative association between leaf miner abundance and oak powdery mildew, consistent with antagonism between oak damage agents. Synthesis. Overall, our study illustrates the importance of considering both tree diversity and composition (neighbour identity) in designing forests more resistant to pest and pathogen damage.
Highlights
The importance of biodiversity for ecosystem functioning and services is being increasingly recognized globally (Cardinale et al, 2012; Isbell et al, 2017; van der Plas et al, 2016)
Separate from the effect of tree species richness, the presence of birch in a plot increased damage by leaf chewers and powdery mildew, but lowered leaf miner damage, suggesting additional tree neighbour identity effects potentially linked to modulation of microclimate
Planting mixed species forests is often reported as reducing forest insect damage compared to monocultures, which has been supported by meta-analyses of evidence from planted and natural forests, and biodiversity-ecosystem functioning experiments (Castagneyrol, Jactel, Vacher, Brockerhoff, & Koricheva, 2014; Jactel & Brockerhoff, 2007)
Summary
The importance of biodiversity for ecosystem functioning and services is being increasingly recognized globally (Cardinale et al, 2012; Isbell et al, 2017; van der Plas et al, 2016). Previous studies showed mixed forests do not always show associational resistance to pests and pathogens (Kambach, Kühn, Castagneyrol, & Bruelheide, 2016; Schuldt et al, 2010), suggesting that associational effects may be context dependent, for instance, on abiotic conditions (Jactel, Poeydebat, van Halder, & Castagneyrol, 2019). Reducing plant apparency as a result of mixing oak with a much taller broadleaved species (birch) lowers mildew and herbivore damage more than predicted just from increasing tree species richness (impacts of neighbour identity as well as diversity); 3. Drought stress could alter the strength and direction of tree diversity effects on both insect herbivory and powdery mildew infection By considering both the abiotic context of tree diversity effects, and the potential for multi-trophic interactions, our work extends our current understanding of the mechanisms of tree diversity effects on insects and pathogens
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